1. Field of the Invention
The present invention relates to a variable valve device for an internal combustion engine, which is switchable between at least two modes by control of a variable valve mechanism with an oil control valve.
2. Description of the Related Art
Examples of an automobile-mounted reciprocating engine (internal combustion engine) having plural cylinders include an engine which uses a variable valve device to switch to a running mode in which cylinders are partially suspended, i.e., a cylinder suspension mode in order to balance improvement of output characteristics and improvement of fuel consumption, when the engine becomes a stable running condition in which large output is not required for the engine. For example, Jpn. Pat. Appln. KOKAI Publication No. 2005-90408 discloses this kind of engine.
Usually, a structure in which a hydraulic type variable valve mechanism and an oil control valve for cylinder suspension mode are provided in an engine cylinder head is used for the switching. In the structure, a hydraulic variable valve mechanism is switchable to a cylinder suspension mode in which the cylinders are partially suspended, and the oil control valve for cylinder suspension mode switches the variable valve mechanism to the cylinder suspension mode.
When the engine enters a predetennined cylinder suspension running range, the oil control valve for cylinder suspension mode is operated. Engine oil is supplied to the variable valve mechanism through a control valve from a lubricating oil pump mounted on an engine body. Then, the variable valve mechanism is driven by an oil pressure of the engine oil, and lifts (open and close) of an inlet valve and an exhaust valve of the cylinder is suspended (cylinder suspension mode).
Because a lubricating oil pump mounted on an engine body is usually driven by a crankshaft, an oil pressure of engine oil discharged from the lubricating oil pump depends on engine revolution speed. The oil pressure generated by the lubricating oil pump becomes low when the engine revolution speed is decreased, and the oil pressure becomes high when the engine revolution speed is increased.
When the engine switches to the cylinder suspension mode, components of the variable valve mechanism are driven against a force applied from the outside, and thus, an oil pressure necessary for the driving is required.
Therefore, in the lubricating oil pump, the oil pressure necessary for the switching of the variable valve mechanism is not sufficiently secured in the low engine revolution speed range. For this reason, the running in the cylinder suspension mode is hardly performed in the low engine revolution speed range, and consequently, a cylinder suspension running range in which the running is performed in the cylinder suspension mode is significantly restricted.
Some of engines include in a cylinder head, a variable valve mechanism, an oil control valve for cylinder suspension mode, and a camshaft-driven oil pump which is an oil pump dedicated to the switching to the cylinder suspension mode independently of the lubricating oil pump. As a result, an oil pressure necessary for the switching of the variable valve mechanism is secured even in the low engine revolution speed range.
In a variable valve device with the oil pump dedicated to the switching to the cylinder suspension mode, a frequency of the switching to the cylinder suspension mode is increased compared with the use of the lubricating oil pump which is already installed in the engine. Particularly, when the dedicated oil pump is used, an opportunity to perform the running in the cylinder suspension mode is increased because the cylinder suspension mode running in the low engine revolution speed range can be performed as well as idling in the cylinder suspension mode.
Consequently, the use of the oil pump for cylinder suspension mode increases the necessity to perform the maintenance to the oil control valve for cylinder suspension mode with the oil pump for cylinder suspension mode, while the oil control valve for cylinder suspension mode conventionally has the low maintenance frequency. Therefore, importance of the maintenance is increasing in the oil control valve for cylinder suspension mode.
However, in the structure in which the oil control valve for cylinder suspension mode and the oil pump for cylinder suspension mode are accommodated in the cylinder head, the maintenance cannot be performed in the oil control valve for cylinder suspension mode unless a rocker cover which covers an upper portion of the cylinder head is detached. This causes a trouble with the maintenance work of the oil control valve for cylinder suspension, in which the maintenance frequency is increased.